U.S. patent number 5,815,919 [Application Number 08/511,056] was granted by the patent office on 1998-10-06 for process for producing an interconnect structure on a printed-wiring board.
This patent grant is currently assigned to International Business Machines Corporation. Invention is credited to Tohru Nakanishi, Hideo Ohkuma.
United States Patent |
5,815,919 |
Nakanishi , et al. |
October 6, 1998 |
Process for producing an interconnect structure on a printed-wiring
board
Abstract
To prevent solder bridges that will easily occur in soldering an
element whose leads are narrow in spacing. In a solder resist,
there is provided an excess-solder absorbing region in which solder
resist is not provided, between an end of a solder pad and the
distal end of a lead. When a great deal of solder paste is applied
to the solder pad for soldering the lead, an excess of solder
melted due to the heat of a heater, flows along the lead and into
the excess-solder absorbing region toward the distal end of the
lead. Since the excess of solder does not flow across the solder
resist provided between the leads, no solder bridge is formed.
Inventors: |
Nakanishi; Tohru (Shiga-ken,
JP), Ohkuma; Hideo (Shiga-ken, JP) |
Assignee: |
International Business Machines
Corporation (Armonk, NY)
|
Family
ID: |
16117571 |
Appl.
No.: |
08/511,056 |
Filed: |
August 3, 1995 |
Foreign Application Priority Data
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|
|
Aug 3, 1994 [JP] |
|
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6-182395 |
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Current U.S.
Class: |
29/840;
228/180.1 |
Current CPC
Class: |
H05K
3/3452 (20130101); H05K 3/3421 (20130101); H05K
2201/0989 (20130101); Y02P 70/50 (20151101); H05K
2203/0195 (20130101); H05K 2201/10689 (20130101); H05K
2201/10424 (20130101); Y10T 29/49144 (20150115); H05K
2201/10681 (20130101) |
Current International
Class: |
H05K
3/34 (20060101); H05K 003/34 () |
Field of
Search: |
;29/840
;228/180.1,180.21 |
Foreign Patent Documents
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557081 |
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Aug 1993 |
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EP |
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2487153 |
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Jul 1991 |
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FR |
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1251788 |
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Oct 1989 |
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JP |
|
2-69992 |
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Mar 1990 |
|
JP |
|
3-062599 |
|
Mar 1991 |
|
JP |
|
4-148587 |
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May 1992 |
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JP |
|
4-336490 |
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Nov 1992 |
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JP |
|
5-152734 |
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Jun 1993 |
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JP |
|
6-53643 |
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Feb 1994 |
|
JP |
|
6-61632 |
|
Mar 1994 |
|
JP |
|
2258183 |
|
Feb 1993 |
|
GB |
|
Other References
IBM Tech. Disclosure Bulletin vol. 27, No. 48 Sep. 1984, pp.
2609-2610 "Enhanced Printed Circuit Board Design"..
|
Primary Examiner: Arbes; Carl J.
Attorney, Agent or Firm: Whitham, Curtis, Whitham &
McGinn Samodovitz; Arthur J.
Claims
We claim:
1. A process for producing interconnect structures, comprising:
forming a substrate with a dielectric surface and an external
surface wiring layer including rows of solder pads adjacent to
respective component mounting sites, for connecting components;
depositing a layer or organic solder resist on the external surface
extending between the pads in the rows; and
providing windows in the resist which expose the soldering pads and
define an excess solder region on the opposite side of the pad from
the respecting mounting site in which region the solder resist is
less thick than elsewhere on the surface and which region is wider
than the gap between pads in the row, and providing excess solder
regions that connect together along a row beyond the solder pad
ends opposite from the mounting site.
2. The invention of claim 1 in which:
there is essentially no resist in the excess solder region; and
providing windows includes providing a gap between the pad and the
edge of the window, in which there is essentially no resist and
which gap extends on each edge of the pad between pads in the row
and which is narrower then the excess solder region.
3. The invention of claim 1 in which:
depositing solder resist includes depositing the resist on the
surface thicker than the thickness of the solder pads; and
the excess solder region is wider than any extension of the leads
past the solder pad edges opposite from the mounting site, so that
the ends of the leads do not rest on the solder resist.
4. The invention of claim 1 in which the solder resist is deposited
between the rows of pads and between the rows and component
sites.
5. The invention of claim 1 in which providing windows includes
providing excess solder regions larger than about 50% of the area
of the solder pads.
6. The invention of claim 1 in which the solder pads are formed
with a spacing at about 0.3 mm or less between centers.
7. A process for producing circuit board assemblies,
comprising:
forming a substrate with a dielectric surface and an external
surface wiring layer including rows of solder pads adjacent to
respective component mounting sites, for connecting components;
depositing a layer or organic solder resist on the external surface
extending between the pads in the rows;
providing windows in the resist which expose the soldering pads and
define an excess solder region on the opposite side of the pad from
the respecting mounting site in which region the solder resist is
less thick than elsewhere on the surface and which region is wider
than the gap between pads in the row;
screening solder paste onto the pads;
mounting a component at the component mounting site with leads
extending on the solder paste on the pads, wherein said component
includes tape extending across the leads; and
holding the lead down onto the pads while reflowing the solder to
form a solder joint between the pad and lead.
8. The invention of claim 7 in which reflowing the solder includes
pressing a hot bar against the leads in the rows to solder the
leads to the pads.
9. The invention of claim 7 in which the component is mounted with
the leads extending past the solder pad edges opposite from the
mounting site and during reflowing the solder forms fillets between
the bottoms of the leads and such opposite edges of the solder
pads.
10. A process for producing circuit board assemblies,
comprising:
forming a substrate with a dielectric surface and an external
surface wiring layer including rows of solder pads adjacent to
respective component mounting sites, for connecting components;
depositing a layer or organic solder resist on the external surface
extending between the pads in the rows;
providing windows in the resist which expose the soldering pads and
define an excess solder region on the opposite side of the pad from
the respecting mounting site in which region the solder resist is
less thick than elsewhere on the surface and which region is wider
than the gap between pads in the row, and providing excess solder
regions that connect together along a row beyond the solder pad
ends opposite from the mounting site;
screening solder paste onto the pads;
mounting a component at the component mounting site with leads
extending on the solder paste on the pads; and
holding the lead down onto the pads while reflowing the solder to
form a solder joint between the pad and lead.
Description
FIELD OF THE INVENTION
The present invention relates to printed-wiring boards and mounting
structures and, more particularly, to preventing solder bridges on
such boards that tend to form when the spacing of the leads of a
surface-mounted element is narrow. The invention also relates to
mounting structures which are capable of preventing solder bridges
that tend to form when leads are repaired with solder.
BACKGROUND
As seen from the expansion in the market for notebook personal
computers, the high-density integration of components has
accelerated in recent years. Mounting boards are no exception. How
components are mounted in a narrow space and how reliability is
maintained are becoming very important considerations.
As the spacing between leads of an element becomes narrower as is
recently the trend, the formation of solder bridges has become
easier and yield is sometimes reduced because of the labor required
for repairing solder bridges.
It is certain that such high-density integration of components will
continue from now on and will be further accelerated. In other
words, it may safely be said that the occurrence of solder bridges
is becoming easier and easier.
Note that various methods for preventing solder bridges have been
proposed. In a printed-wiring board disclosed in Japanese published
utility patent application no. 5-291735, in order to prevent solder
bridges, a solder resist is provided between lands, and on that
solder resist there is provided another solder resist, and this way
the height is increased by forming the solder resists in two
layers. This method, however, has the disadvantage that the number
of steps for forming solder resist is increased and therefore the
cost is increased.
In a printed-wiring board disclosed in Japanese published utility
patent application no. 60-143690, one solder resist covers a
circuit for wiring and another solder resist is provided between
the wiring circuit and a pad for surface-mounting. However, in this
printed-wiring board, the height of the pad for surface-mounting
and the height of the solder resist are the same and, when there is
excess melted solder, there is a high possibility that solder
bridges will occur.
In a printed-wiring board disclosed in Japanese published utility
patent application no. 60-143690, lands are provided on the sides
of foot patterns, and resists are provided around the foot patterns
so that they are formed on the lands. Therefore, on the upper
surfaces of the lands there are formed resist layers (banks) higher
than the foot patterns, and solder bridges are prevented by this
resist layer. However, the resist layers higher than the foot
patterns are noncontinuous and, when there is an excess of melted
solder, there is the possibility that the melted solder flows
between the banks and causes solder bridges.
In a printed-wiring board disclosed in Japanese published utility
patent application no. 59-148388, an ultraviolet-hardened solder
resist is provided around a land of an element to which solder is
applied, to prevent solder bridges. However, in this printed-wiring
board, the height of the solder resist is lower than that of the
land and, when there is an excess of melted solder, there is a high
possibility that solder bridges will occur.
In a printed-wiring board disclosed in Japanese published utility
patent application no. 4-359590, a photo-hardened photosolder
resist is applied to a circuit board, small spaces between
conductive patterns are collectively covered with an opaque mask
film, and the photosolder resist of the opaque portion is exposure
hardened by the transmission of light being transmitted through and
scattered light. However, the height of the photosolder resist is
lower than that of the conductive pattern and, when there is excess
melted solder, there is a high possibility that solder bridges will
occur.
In a printed-wiring board disclosed in Japanese published utility
patent application no. 2-183595, a solder pad or an opening of a
solder resist is formed into a wedge shape to prevent solder
bridges. However, when there is excess melted solder, there is a
high possibility that the melted solder will flow across the solder
resist between the solder pads and cause solder bridges.
In a printed-wiring board disclosed in Japanese published utility
patent application no. 62-74365, in order to prevent solder
bridges, a solder resist is provided between lead patterns,
insulation coating is printed on the solder resist, and a solder
dam with a high surface is formed. This method, however, has the
disadvantage in that the number of steps for printing insulation
coating is increased, so the cost is increased.
In a printed-wiring board disclosed in Japanese published utility
patent application no. 1-162279, in order to prevent solder
bridges, a solder resist is provided between lead patterns,
insulation coating is printed on the solder resist, and a solder
barrier is formed. This method, however, has the disadvantage that
the number of steps for printing insulation coating is increased,
so the cost is increased.
In a printed-wiring board disclosed in Japanese published utility
patent application no. 2-89878, a solder resist is formed on a
circuit board having a plurality of solder pads, and the solder
resist covers the edge of each pad and has a plurality of windows
through which the centers of the pads are exposed. However, when
there is an excess of solder melted, there is a high possibility
that the melted solder will flow across the solder resist between
the pads and causes solder bridges to occur.
In view of the foregoing, it is an object of the present invention
to provide a printed-wiring board which is capable of preventing
the solder bridges that will easily occur in soldering an element
in which the spacing of the leads is narrow, without increasing the
number of manufacturing steps, and a mounting structure which is
capable of preventing solder bridges that will occur when leads are
repaired with solder.
SUMMARY OF THE INVENTION
The invention is a printed-wiring board for mounting an element
having a plurality of long leads and comprises: a circuit board; a
plurality of solder pads for placing and soldering the leads which
is provided on the circuit board; a solder resist for preventing
the bonding of melted solder which is provided on at least a
portion on the circuit board between the solder pads and is also
provided on a predetermined portion on the circuit board excluding
the solder pads; and an excess-solder absorbing region which is
provided on the opposite side of the solder pad on the circuit
board to the element and in which no solder resist is provided so
that an excess of melted solder produced in soldering can escape
from the solder pads and the leads.
In one preferred embodiment of the invention, the remaining portion
of an opening of the solder resist excluding the solder pad is
sized so that it can absorb the excess of melted solder, and the
opening includes the excess-solder absorbing region and the solder
pad is exposed through the opening.
In a further preferred embodiment of the invention, when the lead
is placed on a predetermined position of the solder pad, the
excess-solder absorbing region is opposed to the distal end side of
the lead more than to the solder pad.
In an even further preferred embodiment of the invention, a length
of the excess-solder absorbing region measured from its one end
remote from the element to one end of the solder pad remote from
the element is greater than an allowable error dimension of
printing registration of the solder resist added to a length of a
portion of the lead extending from the solder pad to one end of the
lead remote from the element as the lead is placed on the
predetermined position of the solder pad.
In yet another preferred embodiment of the invention, the element
is surface-mounted by soldering the leads of the element to the
solder pads of the printed-wiring board.
In the printed-wiring board of the invention, a predetermined
amount of solder paste is applied to a plurality of solder pads by
screen printing, etc. Next, an element having a plurality of long
leads is disposed in a predetermined position of the printed-wiring
board so that each lead is opposed to each solder pad. In this
state, by pressing the plurality of leads at the same time by a
heater, solder is melted and the leads are soldered to the solder
pads.
When there inadvertently is is a great deal of solder paste
applied, an excess of melted solder is generated due to the
pressing of the heater, but this excess of melted solder flows
along the lead (mainly the lower surface) into the excess-solder
absorbing region provided on the side of the solder pad remote from
the element. Therefore, solder bridges that will be caused by an
excess of solder can be prevented with reliability.
In the printed-wiring board of this invention having the solder
resist opening sized to absorb excess melted solder since the
remaining portion of an opening of the solder resist excluding the
solder pad is sized so that it can absorb the excess of melted
solder, the excess of melted solder can be absorbed by the
remaining portion. The opening includes the excess-solder absorbing
region and the solder pad is exposed through the opening.
In the printed-wiring board of this invention having the
excess-solder absorbing region opposed more to the distal end side
of the lead solder than to the solder pad, the distal end of the
lead projecting from the solder pad is opposed to the excess-solder
absorbing region. For this reason, when the lead is pressed from
above toward the solder pad by the heater, the lower surface of the
distal end of the lead can be prevented from contacting the solder
resist. For this reason, there is no possibility that the lead is
bent backward and the distal end of the lead is lifted up from the
printed-wiring board.
In the printed-wiring board of this invention having the length of
the excess-solder absorbing region greater than an allowable error
dimension of printing registration of the solder resist added to
the length of an extending portion of the lead even if there
occurred an error in position between the solder pad and the solder
resist to be printed, the lower surface of the distal end of the
lead would not contact the solder resist, because the length of the
excess-solder absorbing region measured from its one end remote
from the element to one end of the solder pad remote from the
element is greater than an allowable error dimension of printing
registration of the solder resist added to a length of a portion of
the lead extending from the solder pad to one end of the lead
remote from the element as the lead is placed on the predetermined
position of the solder pad.
In the mounting structure of this invention having the element
surface-mounted by soldering of its leads to the printed-wiring
board solder pads excess-solder, since the element is
surface-mounted by soldering the lead to the solder pad adjacent to
the excess-solder absorbing region, an excess of melted solder
produced in the solder repair of the lead is absorbed by the
excess-solder absorbing region and therefore solder bridges can be
prevented.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a printed-wiring board and an
element of a first embodiment of the present invention before
soldering;
FIG. 2 is a plan view of the invention showing the leads of the
element soldered on the solder pads;
FIG. 3 is a longitudinal sectional view taken along line 3--3 of
FIG. 2;
FIG. 4 is a perspective view of a printed-wiring board and an
element of a second embodiment of the present invention before
soldering;
FIG. 5 is a perspective view of a printed-wiring board and an
element of a third embodiment of the present invention before
soldering;
FIG. 6 is a perspective view of a printed-wiring board and element
before soldering;
FIG. 7 is an enlarged view of the solder pad of the printed-wiring
board of FIG. 6;
FIG. 8 is a perspective view showing the soder heater and a
resulting solder bridge formed in printed-wiring board of FIG.
6;
FIG. 9 is a sectional view of a pad of FIG. 8 taken in the
longitudinal direction of a lead showing the soldered state as the
center of a solder pad was pressed by a heater;
FIG. 10 is a sectional view of the pad of FIG. 8 taken in the
longitudinal direction of the lead showing the soldered state as
the portion of the solder pad closer to the distal end of the lead
than to the center was pressed by a heater;
FIG. 11 is a perspective view of the vicinity of the solder pad of
FIG. 8 showing the state in which there is an error in position
between the solder pad and the solder resist;
FIG. 12 is a sectional view of FIG. 8 taken in the width direction
of the lead showing the state in which a solder bridge occurred as
the lead was soldered to the solder pad shown in FIG. 11;
FIG. 13 is a perspective view showing a TCP element and a
printed-wiring board of the invention; and
FIG. 14 is a sectional view taken in the longitudinal direction of
the lead of FIG. 13 in the vicinity of the solder pad as the TCP
element was soldered to the printed-wiring board.
DESCRIPTION OF LABELS IN THE DRAWINGS
10 printed-wiring board
12 element
14 lead
16 circuit board
20 solder resist
22 solder pad
24 excess-solder absorbing region
DETAILED DESCRIPTION OF THE INVENTION
To surface-mount an element 104 (e.g., an LSI chip) having a
plurality of leads 102 using solder, a plurality of solder pads 106
are arranged on a printed-wiring board 100 at predetermined
intervals corresponding to leads 102, as shown in FIG. 6. To avoid
electrical contact between adjacent leads 102 through solder
bridges when soldering multiple-lead element 104, solder resist
(portion shaded using dotted lines) 108 is provided around solder
pads 106, as is generally known.
For example, as shown in FIG. 7, the size of solder pad 106 for
soldering lead 102 (having a width of 0.3 mm and a thickness of
0.15 mm) is 1.0 mm in dimension L1 in the longitudinal direction of
lead 102 and 0.35 mm in dimension L2 in the width direction.
A gap is also provided between solder pad 106 and solder resist 108
which defines the opening in solder resist 108. Width W4 of this
gap is about 50 .mu.m. The gap provides for positional tolerance
between the opening of solder resist 108 and solder pad 106.
In soldering such elements 104, solder paste is applied to solder
pads 106 of printed-wiring board 100 by screen printing, or another
process. Leads 102 are placed on the solder paste on the pads and
heater 110 extending in the direction of the arrangement of leads
102, is pressed onto leads 102. A plurality of leads 102 are
soldered at the same time as shown in FIG. 8. At this point, when
heater 110 presses the center of solder pad 106, as shown in FIG.
9, solder fillet 112 is formed with a symmetrical shape with
respect to solder pad 106 in the longitudinal direction of lead
102, but when heater 110 presses one end of solder pad 106 remote
from element 104, as shown in FIG. 10, solder fillet 112 is not a
symmetrical shape in the longitudinal direction of lead 102.
In general, the position at which heater 110 is applied to lead 102
is preferably a position as far away as possible from element 104
to make the thermal and mechanical influence on element 104 as
small as possible and to maintain the flexibility of lead 102 after
soldering. That is, for the flexibility of lead 102 to be
maintained, if the outer end of solder fillet 112 is assumed to be
fulcrum A, dimension S from fulcrum A to element 104 is preferably
longer (FIGS. 9 and 10). Therefore, for the shape of solder fillet
112, the asymmetrical shape shown in FIG. 10 is preferable to that
shown in FIG. 9, and the position at which heater 110 is applied to
lead 102 is normally a position more remote from element 104 than
from the center of solder pad 106, as shown FIG. 10.
Incidentally, when solder paste is applied by screen printing, the
errors of the thickness of the solder paste are about plus or minus
50% or so. As shown in FIG. 7, even if the center of solder pad 106
and the opening of solder resist 108 were aligned with each other
(even if the positional relationship were the center of the
standard), solder bridge 114 is sometimes formed when excess solder
is applied, as shown in FIG. 8. This bridge is normally formed on a
side more remote from element 104 of solder pad 106 than from the
position of heater 110. Solder resist 108 is also normally formed
by screen printing, so a fluctuation in the thickness is great and
an error in the position of the printing also occurs.
If an error has occurred in position between solder pad 106 and
solder resist 108 (but, this error in position is within the
standard), as shown in FIG. 11, solder 116 melted flows to the side
of an adjacent lead 102 and solder bridge 114 is easily formed, as
shown in FIG. 12, even if the amount of solder applied were
appropriate.
Also, if the thickness of solder resist 108 becomes thinner, melted
solder will easily flow across solder resist 108 and formation of
solder bridge 114 may result.
Further, if heater 110 is moved farther from element 104, the
probability of forming solder bridge 114 will become higher. This
is because, by moving heater 110 away from element 104, heater 110
is moved toward point B of solder resist 108, as shown in FIG. 10,
and the lower surface of lead 102 contacts point B when lead 102 is
pressed down by heater 110 and therefore excess solder melted is
prevented from being bonded beyond point B (when the heater is
pressed at a position not so close to point B, the melted solder
can flow beyond point B along the lower surface of lead 102) and,
consequently, the melted solder easily flows to the side of lead
102.
Also, when lead 102 is pressed down with heater 110, lead 102 bends
backward because of the repulsion of corner B of solder resist 108,
and the distal end of lead 102 is lifted up from printed-wiring
board 100 (FIG. 10). The bending of lead 102 becomes greater and
greater, as the pressing position of heater 110 comes closer to the
distal end of lead 102. When such bending of lead 102 became great,
it is conceivable that the distal end of lead 102 will contact
other components, when printed-wiring boards 100 are stacked or
when printed-wiring board 100 comes closer to other components.
Embodiments of a printed-wiring board of the present invention will
be described according to FIGS. 1 through 8.
First Embodiment
As shown in FIG. 1, plurality of leads 14 project from the side of
element 12 (e.g., an LSI or IC chip) that will be mounted on
printed-wiring board 10. As shown in FIGS. 2 and 3, in this
embodiment, thickness t1 (FIG. 3) of lead 14 is 0.15 mm, width W1
(FIG. 2) is 0.3 mm, and pitch P (FIG. 2) is 0.6 mm.
Printed-wiring board 10 is formed on circuit board 16 with pattern
18 of a conductor, such as copper leaf (with a thickness 30 .mu.m
in this embodiment).
As shown in FIG. 2, solder pad 22 for soldering lead 14 is provided
at a predetermined position in pattern 18. Solder pad 22 is a long
rectangular shape in the longitudinal direction of lead 14 and, in
this embodiment, longitudinal dimension L1 in a direction in the
longitudinal direction of lead 14 is 1.0 mm and lateral dimension
L2 in the width direction of lead is 0.35 mm. Width W2 of wiring
portion 18A of pattern 18 is 0.1 mm in this embodiment.
Solder resist 20 is provided on circuit board 16 so that it covers
a predetermined region excluding solder pad 22 of pattern 18. In
this embodiment, thickness t2 (FIG. 3) of solder resist 20 is 30 to
40 .mu.m.
As shown in FIGS. 1 and 2, a gap is provided between the
longitudinal side and solder resist 20. This gap is provided for
absorbing an error in position between solder pad 22 and solder
resist 20 and, in this embodiment, width dimension W4 of the gap is
set to 50 .mu.m.
On the side of the distal end of lead 14 extending from end 22A of
solder pad 22, there is provided an excess-solder absorbing region
24 in which solder resist 20 is not provided.
This excess-solder absorbing region 24 is greater than solder pad
22 by a predetermined dimension. For the size of the excess-solder
absorbing region 24 of this embodiment, the width dimension
(W4.times.2+L2) in a direction in the width direction of lead 14 is
0.45 mm and dimension L4 in a direction in the longitudinal
direction of lead 14 is 0.5 mm.
The operation of this embodiment will next be described. First, a
predetermined amount of solder paste (not shown) is applied to
solder pad 22 of printed-wiring board 10 by screen printing.
Next, element 12 is positioned into a predetermined position on
printed-wiring board 10 so that each lead 14 is opposed to solder
pad 22.
Next, leads 14 are pressed from above at the same time with heater
26. Therefore, as shown in FIG. 3, lead 14 is bent downward by
heater 26 and also the solder paste is melted. After 2 to 3
seconds, the electricity to heater 26 is stopped, cooling air is
applied toward lead 14, and solder 28 is hardened, so solder pad 22
and lead 14 are connected (soldered). Thereafter, heater 26 is
moved upward.
Since the distal end of lead 14 corresponds to the excess-solder
absorbing portion 24 and the lower surface of lead 14 is by no
means brought into contact with solder resist 20, there is no
possibility that lead 14 is bent backward when pressed by heater 26
and the distal end of the lead is bent in a direction away from
printed-wiring board 10. Therefore, the distal end of lead 14 does
not contact, for example, any other component (not shown) over the
printed-wiring board 10. Also, even if a great deal of solder paste
were applied to solder pad 22, an excess of solder 28 melted due to
the heat of heater 26 would flow along lead 14 and into the
excess-solder absorbing region 24 toward the distal end of lead 14
(see right-hand lead 14 shown in FIG. 2, note that in left-hand
lead 14 the amount of solder is almost an appropriate amount).
Therefore, there is no possibility that an excess of solder 28
melted rises above the surface of solder resist 20 and flows across
solder resist 20 between leads 14 to form a so-called solder
bridge.
For the size of excess-solder absorbing region 24, a conceivable
maximum amount of solder paste is applied to solder pad 22, a
lowest size at which solder bridges do not occur is measured by
actually soldering before mass production, and then the size of
region 24 is preferably to be set to a sufficient size greater than
the area of the measured lowest size. Note that, as a rough
criterion, the area of excess-solder absorbing region 24
{=L4.times.(W4.times.2+L2)} is preferably more than 50% of the area
of solder pad 22. This is a rough criterion, however, so the area
of excess-solder absorbing region 24 changes depending on how much
excess solder is produced in the manufacturing process.
It is to be noted that, precisely speaking, the volume of a space
that the volume of pattern 18 in the opening of solder resist 20
(almost equal to L1.times.L2.times.t3) is subtracted from the
volume of the opening of solder resist 20
{=(W4.times.2+L2).times.L3.times.t2}, is a volume that can absorb
an excess of solder 28 with reliability.
Thus, in printed-wiring board 10 of this embodiment, even if there
were a great deal of solder, solder bridges could be reliably
prevented because an excess of solder melted can escape into
excess-solder absorbing region 24. Also, even if pitch P between
leads 14 becomes narrower in future, it is obvious that an
occurrence of solder bridges can be prevented.
Note that a structure in which element 12 is soldered and mounted
on printed-wiring board 10 is a mounting structure.
Alternately solder (paste) may be applied to solder pad 22 by
electroplating and by electroless plating, but these methods
require complicated equipment and are expensive, as compared to the
screen printing method employed in this embodiment.
Second Embodiment
A second embodiment of the present invention will be described
according to FIG. 4. The same reference numerals will be applied to
the same constitution as the first embodiment and therefore a
description thereof will be omitted here.
In printed-wiring board 10 of this embodiment, solder resist 20
provided between leads 14 terminates short of an end 10A of
printed-wiring board 10. As with the first embodiment, this
embodiment also can prevent solder bridges.
It is to be noted that an end 20A of solder resist 20 provided
between leads 14 is preferably positioned beyond at least solder
pad 22 toward the distal end of lead 14. Also, end 20A of solder
resist 20 is set after it has been proven by experiment that an
excess of solder 28 melted does not contact end 20A.
Third Embodiment
A third embodiment of the present invention will be described
according to FIG. 5. The same reference numerals will be applied to
the same constitution as the first embodiment and therefore a
description thereof will be omitted here.
In this embodiment, there is shown an example of a case where
element 12 (not shown in FIG. 5) is disposed, for example, in the
vicinity of the center of printed-wiring board 10.
This embodiment can also prevent solder bridges if, as a rough
criterion, the area of an excess-solder absorbing region 24 is set
to more than 50% of the area of solder pad 22. Note that this is a
rough criterion, so the area of the excess-solder absorbing region
24 changes depending on how much excess solder is produced in the
manufacturing process.
It is to be noted that solder resist 20 is printed on pattern 18
(see FIG. 2), so a positional error sometimes occurs between solder
pad 22 and solder resist 20. The dimensions of excess-solder
absorbing region 24 are preferably set so that, even if the maximum
positional error within a standard occurs, lead 14 projecting from
solder pad 22 would be opposed to excess-solder absorbing region 24
at all times.
In this embodiment, since the maximum error in position between
solder pad 22 and solder resist 20 is assumed to be 0.1 mm, the
dimensions of the excess-solder absorbing region 24 are preferable
to be set in view of a dimension greater than 0.1 mm.
Fourth Embodiment
A fourth embodiment of the present invention will be described
according to FIGS. 13 and 14. The same reference numerals will be
applied to the same constitution as the first embodiment and
therefore a description thereof will be omitted here.
Some of elements 12 are called a tape carrier package (TCP). When
this type of element 12 is soldered, leads 14 of element 12 are
easily damaged and deformed because they are very thin.
For this reason, reinforcing tape 30 is attached to the distal ends
of leads 14 to prevent the deformation of leads 14, as shown in
FIG. 13.
This reinforcing tape 30 is normally made from a polyamide
material, poor in heat conduction, and reduces the rate of heat
conduction from heater 26 to solder, and not good for soldering, so
it is as far as possible away from solder so that it does not
contact heater 26.
In a conventional printed-wiring board of this kind, there was the
problem that excess melted solder is blocked by the reinforcing
tape and therefore flows to an adjacent lead along the reinforcing
tape, so solder bridges are easily formed. However, since in
printed-wiring board 10 of the present invention an excess of
melted solder can flow into excess-solder absorbing region 24,
solder bridges can be prevented even if reinforcing tape 30 were
provided on leads 14.
It is to be noted that the relationship of dimensions in the
above-described embodiment is merely an example. Needless to say,
the dimensions of excess-solder absorbing region 24 can be changed
according to the dimensions of lead 14 and solder pad 22.
Also, in a mounting structure in which an element is mounted on the
printed-wiring board described in the above embodiments, soldering
is sometimes performed again in the repair and replacement of
elements. In this case, there is also the possibility that an
excess of melted solder is produced when there is a great deal of
solder. However, in such a case, solder bridges can also be
prevented in element repairs because an excess of melted solder is
absorbed by the excess-solder absorbing region.
As has been described hereinbefore, the printed-wiring board of
this invention has the advantage that, even if there were an excess
of solder, solder bridges could be prevented because melted solder
can flow into the excess-solder absorbing region provided on the
side of the solder pad remote from the element.
The printed-wiring board in a further embodiment of this invention
also has the excellent advantage that all of an excess of melted
solder can be absorbed by the remaining of an opening of the solder
resist excluding the solder pad. The opening includes the
excess-solder absorbing region and the solder pad is exposed
through the opening.
The printed-wiring board in another embodiment of this invention
also has the excellent advantage that the distal end of the lead
can be prevented from being lifted up from the printed-wiring board
when the lead is soldered by the heater, and also the distal end of
the lead can be prevented from contacting other components, because
the distal end of the lead projecting from the solder pad is
opposed to the excess-solder absorbing region.
The printed-wiring board in yet another embodiment of this
invention also has the excellent advantage that, even if there
occurred an error in position between the solder pad and the solder
resist to be printed, the distal end of the lead could be prevented
from being lifted up from the printed-wiring board when the lead is
soldered by the heater and also the distal end would not contact
other components, because the length of the excess-solder absorbing
region measured from its one end remote from the element to one end
of the solder pad remote from the element is greater than the
allowable error dimension of printing registration of the solder
resist added to a length of a portion of the lead extending from
the solder pad to one end of the lead remote from the element as
the lead is placed on the predetermined position of the solder
pad.
The mounting structure in another embodiment of this invention also
has the excellent advantage that an excess of melted solder
produced in the solder repair of the lead is absorbed by the
excess-solder absorbing region and therefore solder bridges can be
prevented because the element is surface-mounted by soldering the
lead to the solder pad adjacent to the excess-solder absorbing
region.
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